Why did Auggie only start 5th grade at the age of ten?

From the text:

Next week I start fifth grade. Since I’ve never been to a real school before, I am pretty much totally and completely petrified. People think I haven’t gone to school because of the way I look, but it’s not that. It’s because of all the surgeries I’ve had. Twenty-seven since I was born. The bigger ones happened before I was even four years old, so I don’t remember those. But I’ve had two or three surgeries every year since then (some big, some small), and because I’m little for my age, and I have some other medical mysteries that doctors never really figured out, I used to get sick a lot. That’s why my parents decided it was better if I didn’t go to school. I’m much stronger now, though. The last surgery I had was eight months ago, and I probably won’t have to have any more for another couple of years.